Blading for axial flow turbo-machine

ABSTRACT

Turbine blading for the rotors of turbo-machines of the axialflow type in which the blades are twisted to allow for a high ratio of external to internal diameter of the flow passageway, the amount of the twist being so selected that it will neither untwist nor increase its twist when subjected to the centrifugal forces which act on the blade when the rotor is operating.

inited States Patent 1 91 Seippel I Dec. 3, 1974 BLADING FOR AXIAL FLOW TURBO-MACHINE [75] Inventor: Claude Seippel, Zurich, Switzerland [73] Assignee: BBC Brown Boveri & Company Limited, Baden, Switzerland [22] Filed: Jan, 12, 1973 [21] Appl. No.: 323,094

[30] I Foreign Application Priority Data Jan. 20, 1972 Switzerland 76772/72 52 us. 01. 416/223, 416/243 2 [51] Int. Cl. F01d 5/14 [58] Field of Search 416/223, 242, 243

[56] References Cited UNITED STATES PATENTS 2,116,055 5/1938 Weichwald 416/223 2,928,653 3/1960 Roberson 416/210 x FOREIGN PATENTS OR APPLICATIONS 1,187,872 3 1959 France 416/240 131,648 9/1919 Great Britain 416 240 614,074 12/1948 Great Britain 416/240 I Primary ExaminerEverette A. Powell, Jr. Attorney, Agent, or Firm-Pierce, Scheffler 8L Parker [57] ABSTRACT Turbine blading for the rotors of turbo-machines of the axial-flow type in which the blades are twisted to allow for a high ratio of external to internal diameter of the flow passageway, the amount of the twist being so selected that it will neither untwist nor increase its twist when subjected to the centrifugal forces which act on the blade when the rotor is operating.

2 Claims, 4 Drawing Figures BLADING FOR AXIAL FLOW TURBO-MACHINE locities which occur along the face of the blade between its root and tip. Experience has shown, however, that such twisted blades will tend to untwist due to the longitudinal pull exerted thereon as a result of the centrifugal forces to which the blade is subjected as it rotates at rather high speeds, with the result that the amount of the twist will differ, depending upon whether the rotor, and hence the blading is rotating, or at standstill. This will result not only in some uncertainty as to the aerodynamic effect of the blading, especially during the startingperiod, and also at lower speeds, but also in the generation of additional stresses which will be superimposed on the alreadyhigh centrifugal as well as bending stresses.

The object of this invention is to provide a twisted blade strurcture in which the amount of twist will remain the same, i.e., both at-standstill and during operation. This objective is attained by selecting the degree vision need be made for any potential changes in shape of the blade during operation. 7

The invention'will become more apparent from the following description of a preferred embodiment thereof and in conjunction with the accompanying drawings wherein:

' FIG. I is aschematic presentation of a twisted blade illustrating the nature of the centrifugal forces acting upon it under dynamic conditions,

FIG. 2 is a view similar to FIG. 1 but wherein the blade is straight; i.e., non-twistedi I FIG. 3 illustrates a blade of the twist type when at rest, i.e., under static conditions; and

FIG. 4illustrates a twisted blade constructed in accordance with the principles of this invention.

With reference now to FIG. I, there is shown the tip 1 which represents a thinsection of a turbine blade having an airfoil profile. The leading and trailing edges of the blade are designated by 2 and 3, respectively. At the center of mass 4, the tip 1 is secured to pin 5 which is located vertically to the axis of rotation 6, and thus in a radial direction. Centrifugal forces acting upon the masses in the center of the mass. exert a pull in the axial direction of the pin and are designated by vector 4-7. The centrifugal forces which originate from the masses at point 2, are depicted by a radial vector 2-8 which intersects the axis 'of rotation 6 vertically. Since point 2 is staggered in the peripheral direction in relation to point 4, the forces 2-8 and 4-7 are not parallel. Vector 2-8 can be resolved into two components, one such component 2-9 being parallel to vector 4-7, and the other component 9-8 perpendicular to the latter andfunctioning in a tangential direction. In the same manner, theradial forces 3-10 can be resolved into two components, one such component 3-11 being parallel to vector 4-7 and the other component I1-l0perpendicular to the latter. Those components which are 'parallel to the vector 4-7 will form a resultant which exerts a purely tensional force upon the pin 5 if, as stipulated, point 4 represents the center of mass for the tip 1. Under the same conditions will the perpendicular vector components exert a purely torsional force onto the pin 5.

FIG. 2 shows a rotor 12 upon which is fastened a circumferential array of blading. However, in order to simplify the disclosure only one such blade 13 has been included. The blade is not twisted about its longitudinal axis and it has a constant cross-sectional profile throughout its entire length. The axis 14 of the center of mass is located radially to the axis of rotation 6. When this blade system is rotated rapidly, the blades will stretch longitudinally and, as explained on the basis of FIG. 1, will twist in the direction of the arrows 15.

FIG. 3 shows a blade which has a sharp twist, the

blade being in the rest, i.e., when the .rotor is at standstill. The forces generated on the turbine blade are not parallel to each other, and are not perpendicular, but rather are inclined to the axis of rotation 6. During operationof the blading, the blades 16 tend to straighten out under the influence of the centrifugal forces, and to assume a position perpendicular to the axis of rotation, thus tending to untwist the blade, in known manner, in the direction of the arrows 17.

In accordance with the novel concept of this invention, the twist imparted to the blade is such that the twistingand untwisting forces just neutralize one another. FIG. 4 illustrates an embodiment of a blade structured in accordance with the invention. The blade, made of metal, is connected to the rotor 12 along the path 19-20-21. It is designedin the form of a control wing where all of the force generating vectors 19-22, 20-23 and 21-24 are located in a radial direction, that is to say, they intersect the axis of rotation 6 at an angle of It will be readily apparent that such a blade structure can not and'will not change its shape by twisting or untwisting when subjected to centrifugal forces during rotation of the rotor.

The invention is not limited to the specific blade design illustrated in FIG. 4 but rather other configurations may be adopted by which the same result can be obtained. For example, in contrast to the blade configuration depicted in FIG. 4, the thickness or the length of the cord of the profile diminishes from within to without.

The principle which determines the extent of the blade twist can be expressed by the equation dB/dr T/P All, I; wherein r is the axial pitch of any blade section on which the following values are based.

B is the angle between the principal inertia axis and the circumferential direction.

dB/dr is the degree of blade twist T is the resultant torsion moment of the blade part from the distance r to the end of the blade P is the resultant tension force of this part of the blade A is the cross-sectional area of the profile, and

I 1 are the maximum and minimum moments of inertia of the blade profile The shape of the blade can deviate somewhat in actual practice from the theoretical shape as defined by the above equation if a slight correction is indicated for purposes of flow-engineering.

I claim:

1. The improvement in turbine blading for the rotor of a turbo-machine of the axial-flow type wherein for the purpose of allowing a high ratio of external to internal diameter of the flow passageway each of the blades is given a twist about its horizontal axis which is determined at least approximately in accordance with the equation dB/dr T/P A/l 1 wherein r is the axial pitch of any blade section on which the following values are based. B is the angle between the principal inertia axis and the circumferential direction.

dB/dr is the degree of blade twist T is the resultant torsion moment of the blade part from the distance r to the end of the blade P is the resultant tension force of this part of the blade A is the cross-sectional area of the profile. and

l 1 are the maximum and minimum moments of inertia of the blade profile thereby maintaining constant the amount of the blade twist under the influence of centrifugal forces acting thereon when the motor is operating.

2. Turbine blading as defined in claim 1 wherein all centrifugal force generating vectors along the surface of the blade between its leading and trailing edges extend in a radial direction from the axis of rotation of the rotor. 

1. The improvement in turbine blading for the rotor of a turbomachine of the axial-flow type wherein for the purpose of allowing a high ratio of external to internal diameter of the flow passageway each of the blades is given a twist about its horizontal axis which is determined at least approximately in accordance with the equation d Beta /dr T/P . A/I1 + I2 wherein r is the axial pitch of any blade section on which the following values are based. Beta is the angle between the principal inertia axis and the circumferential direction. d Beta /dr is the degree of blade twist T is the resultant torsion moment of the blade part from the distance r to the end of the blade P is the resultant tension force of this part of the blade A is the cross-sectional area of the profile, and I1, I2 are the maximum and minimum moments of inertia of the blade profile thereby maintaining constant the amount of the blade twist under the influence of centrifugal forces acting thereon when the motor is operating.
 2. Turbine blading as defined in claim 1 wherein all centrifugal force generating vectors along the surface of the blade between its leading and trailing edges extend in a radial direction from the axis of rotation of the rotor. 